Preface - Since the mammoth El Niño event began around
June, 1997, we have been amazed at the media coverage El Niño has garnered. It
has been our observation, however, that much of the play in the media has
ranged from superficial to misleading – and people still seem to be greatly
confused. In what follows, we list some of the questions we have received from
the ordinary "man in the street", along with our responses. This
material does not necessarily reflect the official stance of the National
Oceanic and Atmospheric Administration (NOAA).

Reader input in the form of questions
or comments is welcome You may contact us at the above snail-mail address or
send us e-mail at: David.Enfield@noaa.gov.

El Niño is an unusual warming of the
tropical Pacific Ocean that occurs irregularly at about 3-6 year intervals in
response to large scale weakenings of the trade winds that normally blow
westward from South America toward Asia. Normally, the trade winds produce cool
surface water in the eastern Pacific, through evaporation and the upwelling of
colder water from below the surface. Simultaneously, they "corral"
warm surface waters over in the far western Pacific. As the trade winds weaken,
so does the containment of the warm water in the west and the maintenance of
the coolness in the east. As a result, relatively warm water becomes ubiquitous
all across the Pacific from New Guinea to South America. Although the immediate
cause (wind weakening) is known and scientists have made much progress in
understanding the phenomenon, the exact nature of the processes that govern its
repetitive cycle are still not certain.

Most
definitely. El Niño may be thought of as one of Earth's standard mechanisms for
getting heat from the tropics (where more comes in from the sun than goes out)
to the polar regions (where more heat returns to space than comes in). Ordinary
winter storms also do this. Without these poleward transports of heat, the
planet would be an unbearable hothouse in the tropics or too cold for
habitation toward the poles. In the years between ENSO events, excess heat
accumulates in the tropics and then gets "exported" during El Niño.
It's somewhat like the accumulation of winter snow on a steep mountain slope.
The snow cannot accumulate indefinitely, defying gravity; inevitably it must
give way to avalanches. What happens during an El Niño "avalanche" is
that a lot of excess heat gets transported poleward, most frequently through
winter storms. That is why places like California and Chile have rougher
winters in conjunction with El Niño.

El Niño is
also responsible for a good deal of diversity in plant and animal life because
the periodic stress it puts on biological systems is a stimulus to the
evolutionary process. Moreover, the affected biota on both land and in the sea
are much more resilient as a result of the need to survive these periodic
upheavals in the environment.

• What is the history behind the term "El Niño"?

Our
first knowledge of this came from Peruvian geographers, who at the end of the
19th century were interested in the unusual climate aberrations that occurred
along the Peru coast in the odd year. They took note of what a knowledgeable
ship captain said about the fishermen in northern Peru, who typically saw a
switch from cold to tropical ocean conditions around Christmas of every year and
attributed this to a southward setting, warm "El Niño current". The
term was an obvious reference to the Christ child. We don't actually know how
mythical this story might be, but that is the tale that got passed on. The
geographers noted that in some years the onset of warm conditions was stronger
than usual and was accompanied by unusual oceanic and climatic phenomena.

Starting
with the arrival of foreign-based scientific expeditions off Peru in the early
20th century, the concept gradually spread through the world's scientific
community that "El Niño" referred to the unusual events. The annual
occurrence was forgotten, although one geographer (Eguiguren) lamented this
inaccuracy. It was separately noted by Sir Gilbert Walker in the 1930's that
notable climate anomalies occur around the world every few years. These were
associated with what he called the Southern Oscillation (SO), a large
fluctuation in atmospheric pressure. In the 1950's, Berlage observed that the
SO-related climate anomalies generally coincided with El Niño occurrences. It
wasn't until about 1960 that scientists came to realize that the warming off
Peru is only part of an ocean-wide perturbation that extends westward along the
equator out to the dateline. About the same time, the noted meteorologist Jacob
Bjerknes proposed that El Niño was just the oceanic expression of a large-scale
interaction between the ocean and the atmosphere and that the climate anomalies
could be understood as atmospheric "teleconnections" emanating from
the warm-water regions along the equator in the mid-Pacific. The catchy term
"El Niño" is frequently abused in the popular vernacular through the
tendency of people to confuse what is essentially an oceanic happening with the
climate anomalies that are associated with it.

Starting
in about 1975, oceanographers and meteorologists began to combine their efforts
to expand and refine the Bjerknes hypothesis by systematically studying the El
Niño and the Southern Oscillation together in what we now call "El Niño–Southern
Oscillation", or ENSO. The advent of powerful computers and
modern measurement systems has caused a rapid acceleration in our understanding
of ENSO, especially since the large event of 1982/83.

• How often do ENSO events occur, and are they always so
strong?

On
average they occur every 3-6 years but only irregularly and not as predictably
as the astronomically controlled tides. As measured by the degree of warming, every
other event tends to stronger or weaker, with the strong ones occurring only at
8 to 15 year intervals. The intervening weak and moderate events do not
typically bring such disastrous consequences. The events of 1982/83 and 1997/98
were unusually strong, equaled historically only by events in the late 1800s.
Really big events like 1982/83 and 1997/98 occur only a few times in a century.

• I've heard about something called "La Niña".
What's that?

"La
Niña" refers to certain years between El Niño events in which most of the El
Niño characteristics are reversed, including the tropical Pacific ocean
temperatures, which are colder than average. Because it is contrary to El Niño
(cold instead of warm) and its climatic effects are frequently opposite as well
(droughts instead of floods, and vice versa), it is given the contrasting
feminine name in opposition to the masculine "El Niño". The way we
characterize ENSO anomalies is to subtract long-term averages from our data.
During the intervening years between El Niño events the equatorial Pacific
Ocean is somewhat cooler than that average, since together with the warm El
Niño events they must sum to the average. So you might say that La Niña is an
inevitable consequence of how we calculate the anomalies. However, in some
years the equator is unusually cold, though never as cold as a strong El Niño
is warm. This prompts some scientists to think that La Niña is a physical
phenomenon in its own right, or at the very least, the opposite phase of an
"ENSO cycle". Scientists have never predicted La Niña and it is
difficult to agree on what constitutes La Niña. La Niña events with significant
impacts are fairly unusual.

No. El Niño has been occurring at least since people started putting
thermometers in the ocean around the middle of the nineteenth century. Moreover,
archived documents left by the Spanish colonists in Peru confirm that El Niño
impacts such as occur now (flooding, marine life disturbances, etc.) have
been felt in Peru ever since the first conquistador (Francisco Pizarro) set
foot there in the early 16th century. And, as far as we can tell
from paleo-climatic indicators such as geological evidence & tree rings,
El Niño has been occurring for at least thousands of years, probably much
as it has during this century. It will probably continue to occur as long
as our climate system works the way it has since the most recent ice sheets
of the late pleistocene receded (i.e., needing to get rid of excess tropical
heat as explained in the question Does El Niño play a special role in Nature?).

• So, did El Niño also occur during the ice ages?

We
don't know yet. The global climate was very different then and the need for a heat-exporting
mechanism may not have existed. As you might guess, however, scientists are
anxious to know this because it will help us to understand how sensitive the
features of our present climate system (such as ENSO) are to significant
changes in the climatic background state. Clearly we are altering our present
climate, so we need to understand what kinds of changes in weather systems
might be expected as a result.

• So, will the global warming due to greenhouse gases
(exhaust from our cars and factories) cause ENSO events to become more frequent
or more severe?

We
don't know that for certain. Some studies suggest this may be true while others
cast doubt on the idea. Part of the problem is that natural changes in the
frequency and intensity of ENSO events have occurred in the last five centuries
for which records exist, and it is hard for us to distinguish those from recent
characteristics that might otherwise be attributed to greenhouse warming. This
is also a subject of great interest in research. Unfortunately, while ENSO
intervals are well matched to the political time scale that governs our
research funding (3-4 years), global warming is not.

Absolutely,
and rather drastically in the case of the stronger events, such as 1997/98. If
all that occurred during an El Niño were a warming of the equatorial ocean
where nobody lives, El Niño would not occupy the public awareness as it now
does.

• El Niño gets talked about in terms of both climate and weather. What is
the distinction between these two things and how does it help us to understand
the effects of El Niño?

"Climate is
what we EXPECT,

weather is what
we GET."

[Robert A.
Heinlein]

To
understand the twin concepts of climate and weather and why they should be
affected by El Niño, think of the atmosphere as a huge pot of fluid (our
atmosphere is a mixture of gaseous fluids, after all) on a stove with heating
elements (hot spots of warm tropical ocean temperatures) located under certain
portions of the pot. This creates a characteristic circulation (climate) in the
pot, with warm fluid rising over the hot spots and moving away toward the
cooler regions where it sinks (the Earth's higher latitudes). The circulation
has embedded turbulence, or eddies (weather), that are inherently less
predictable than the average circulation itself, but nevertheless conform to a
statistically expected behavior (again, climate). As thermal anomalies develop
in ocean temperatures during El Niño, displacing cold water in certain regions,
the distribution and intensity of the hot spots under the pot are changed;
naturally, so too does the circulation pattern in the pot change, along with
the statistical expectations for the turbulence (i.e., the weather). Although
we cannot predict the precise nature of the altered weather far in advance, we
do know more or less how different it will be on average.

"In
response to the more uniform pattern of heating in the tropics during an El
Niño the wintertime jet streams in each hemisphere tend to be more uniform from
east to west and extend farther east than normal. However, the timing, location
and magnitude of the ocean warming varies from one El Niño to the next, which
results in variations in the patterns of tropical rainfall and deep tropical
heating. These conditions contribute to variations in the precise location,
strength and structure of the mid-latitude jet streams over both the North and
South Pacific from one El Niño to the next, and thus to the variability in
weather patterns and storm tracks over North and South America.

"A
second major reason for the variability in weather patterns from one El Niño to
the next is simply that El Niño is not the only factor influencing the weather and
climate. In particular, the atmosphere exhibits considerable variability on
time scales ranging from days to seasons to years, and this variability often
reflects nothing more than the normal chaotic behavior of the atmosphere. This
description is particularly applicable to areas such as eastern North America,
the North Atlantic, Europe, etc., which are heavily influenced by features such
as the North Atlantic jet stream. "

There
will continue to be surprises associated with future El Niño events. Scientists
have really only focused on ENSO as a large scale phenomenon since the mid
1970s. We have not witnessed all the forms these events can take nor have we
recognized all the ways they can affect societies and ecosystems.

•Is the occurrence of a
strong "El Niño" always synonymous with disaster and Armageddon?

"When
interpreting the climate information linked to El Niño it is important to note
that while abnormal temperature and rainfall patterns can and sometimes do
result in severe climate conditions, they do not imply calamitous conditions in
many instances. For example, the above-normal precipitation expected in the
Southwest and southern plains states implies a reduced chance for wintertime
drought such as occurred during November 1995 - May 1996. In Florida the
above-normal rainfall expected this winter indicates reduced chances of
wildfires. In California, the potential impacts from El Niño can be severe.
However, the above-normal rainfall across the state during the 1992-93 El Niño
resulted in an end to severe long-term drought conditions that had persisted
since 1986/87, and to a much-needed replenishment of water reserves."

El
Nino affects marine life mainly through the drastic changes that occur in the
Pacific ocean, especially along the equator and the Pacific coasts of North and
South America. The two principal factors are (1) the intense warming in regions
of normally cool, upwelled water, and (2) the reduction in the supply of high,
subsurface nutrients that normally upwell in the same regions. During El Niño
changes occur in the distribution and abundance of many species. During the
milder El Nino events, the cold-loving Peruvian anchovy becomes scarce off Peru
and more prevalent in the cooler Chilean waters to the south. In some instances
the anchovy has been replaced by population increases of pelagic species that
do better in warmer water, such as sardine and Spanish mackerel. Not only does
the anchovy not like the warmer water, but the associated decrease in nutrients
has a negative impact on the abundance of its principal food source: the
microscopic algae (phytoplankton) that are normally so plentiful along the
productive Peru coast. During strong events many other species are also
affected and changes in species distributions can be seen as far away as the
Gulf of Alaska.

•So, what happens to
marine fauna during the stronger events?

In
general, species that like warmer water become more prevalent in the cool-water
regions off the coasts of North and South America. Both coasts frequently see
increases in certain nearshore benthic (bottom) fauna, such as shrimp and
scallops, which reproduce and survive better in the warm water (they are not
great migrators). Migrating species of offshore pelagic (mid-water) fish, such
as dolphinfish (also known as dorado, or mahi-mahi) typically invade the
normally cool coastal waters in greater numbers; other tropical species,
including popular sportfish like yellowtail, may be found far poleward of their
normal distribution (much to the liking of deep sea fishermen in California).
Cold-water fish such as salmon may be found closer to the poles, migrating from
Oregon-Washington to the Gulf of Alaska. The marine ecosystems of both
continents, from the microscopic phyto- and zooplankton to the largest predator
fishes, may be altered for up to a year during a strong event such as 1997/98.

•What happens to species
that can't migrate, such as corals?

Corals
have a hard time during the more intense events. In 1982/83 (a very strong
event) many of the coral species of the Galapagos Islands were killed in large
numbers. Fifteen years later in 1997/98 many of the previous survivors perished
as well. Many coral species depend on a symbiotic relationship they have with
algae-like species (called zooxanthellae) that live in the gastrodermal tissues
of the corals and increase the availability of food. When the water warms too
much, the zooxanthellae disappear; the white color of the coral skeleton ceases
to be obscured by the darker zooxanthellae and the corals are said to
"bleach". If the waters remain warm for too long, the prolonged
bleaching stresses the coral metabolism to the breaking point and they die. The
good news is that corals are capable of re-establishing themselves in decimated
areas, possibly after several years, by means of immigrating larvae from distant surviving populations.

Oddities and potpourri about El Niño:

• I heard somewhere that El Niño slows down the rotation of
the earth. True?

El Niño
cycles cause a correlated but small fluctuation in the "length of
day" (it gets longer), so yes, this is true. The reason it happens is that
the entire Earth system (solid earth, air and water) must conserve its total
angular momentum (related to the speed of rotation around the earth's axis),
like a spinning top, or a twirling ice skater. During El Niño, the average
eastward speed of the winds around the globe increases, which is related to the
reason California and Chile get more and stronger winter storms (see Does El Niño play a
special role in Nature?).
Since the angular momentum of the air increases, the rotational speed of the
solid earth must decrease, in compensation. Of course, the resulting increase
in the length of the day is very small, so don't worry about arriving early for
your next appointment...

This
complaint came from a Californian as the severe El Niño winter extended into
spring. Starting at the beginning of February, a seemingly endless series of
storms, one after another, assaulted the California coast with flooding,
erosion and misery. Some were stronger than others but all were significant in
comparison with those of a normal winter. Instead of tracking across the
Pacific south of the Aleutians and into the Gulf of Alaska, they crossed Hawaii
straight into California, picking up moisture and energy from the heat-laden
tropical Pacific (El Niño). From there they continued across the desert
southwest, dropping rain and snow from Arizona to Texas, where they picked up
new moisture from the nearby Caribbean and Gulf of Mexico, finally continuing
eastward to drench the southeastern states, frequently spawning tornadoes as
they passed. During the 1982/83 event, a series of 13 storms followed a similar
pattern starting at the beginning of January and did not let up until early
April.

• During an El Niño event, when do the weather problems hit
California, and how longdo they last?

Off
and on throughout the winter, if El Niño is strong then. Then it snows a lot in
the mountains and it rains a lot elsewhere in California. Some of that is
actually good, such as for skiers. In both 1982/83 and 1997/98, the worst
effects were felt after the New Year. Possibly the worst thing that can happen
is in the spring if El Niño is still going strong as occurred in 1983. Then you
can get a rapid wetting and melting of the large snowpack in the western
mountains, producing severe flooding in some areas. This can be very bad. It's
something the folks who manage water supplies (dams and such) have to watch out
for.

• In the summer of 1997 a Pacific hurricane threatened
California and there were predictions that increased hurricane frequency and
intensity could spell disaster, that monsoons would hit northern California,
etcetera.

For
the most part, such talk was unwarranted. First of all, monsoons are not
hurricanes, they are periods of significant seasonal change in tropical weather
and wind direction. All major continents are affected by monsoons, and they may
be stronger or weaker in association with El Niño. However, a hurricane in
California is only remotely possible, even during an El Niño. The possibility
that California could get hit by a major hurricane is very small,
especially during the period that El Niño is expected to produce its strongest
impacts (California's winter).

• Why is the idea of a California hurricane so implausible?

The
ocean temperatures off California – including during a strong El Niño – are so
cold that they would grind down a hurricane to nothing more that a strong,
rainy windstorm. Historical statistics indicate that a major California
hurricane is no more likely in an El Niño year than in a normal year. Such an
event has never occurred since hurricane records began, so we can only guess
that such a thing might happen once in a millennium or so.

• OK, but amid all this talk of California hurricanes, we
also heard TV forecasters saying that El Niño had knocked down the chance of
hurricanes. This confused me!

You
have to keep your oceans straight. In the Pacific, El Niño has little
discernible effect on hurricanes close to the continent. Atlantic hurricanes
are an entirely different matter. There, you see, El Niño almost always reduces the frequency of storm maturation from what it
would otherwise be, just as the TV weathercasters say. This is because El Niño
produces increased wind shear
("scissors effect") over the tropical North Atlantic in the region
where storms born off northwest Africa try to mature into hurricanes. This
shear, or wind difference between the high and low levels of the atmosphere,
tears many of the developing storms apart before they can become serious
threats.

• Where else in the Americas are severe effects felt?

Best
known, of course, are the coastal regions of Ecuador and northern Peru, where
so much rain falls that flooding, landslides, erosion and consequent property
destruction reach disastrous proportions. The moisture causes explosive
infestations of insects and associated problems with public hygiene that favor
the development of water-born and mosquito-vectored diseases (cholera, malaria,
dengue, etc.). In the deserts of Peru and Chile, entire "pampas"
bloom with flowers where nothing but a barren expanse is normally found. In the
far north of Peru, shallow lakes appear on the desert, with ecologies of plants
and fishes that temporarily sustain migrant human populations. Farther south,
both Chile and Argentina, and even southern Brazil, experience severe storms
during the austral winter (June-August). Northeastern Brazil goes through
severe drought in March-May, with consequences for its agriculture. Dry
conditions over much of Central America and northern South America also affect
agriculture and additionally cause deficits of hydroelectric energy and
drinking water.

• What about other regions of the world?

El
Niño impacts are typically quite severe over southeast Asia, including
Malaysia, Indonesia, New Guinea, Borneo and northern Australia. Throughout much
of that region drought is common, agriculture is affected and tropical forest
fires become a problem. Although western Pacific typhoons (overall) are no more
frequent than normal, they tend to affect areas farther to the east, as far as
Tahiti, that are otherwise less affected. To the west there is typically a
failure of the summer (wet) monsoon over the Indian subcontinent, whose
populations depend critically on the monsoon rains. Finally, droughts are known
to occur in southern Africa and northeastern Africa.

• Are the effects of El Niño all bad?

No.
During El Niño winters, for example, Florida receives significantly more rain
than normal, which reduces the risk of wildfires in the spring and early
summer. Reservoirs fill and skiers and sport fisherman are happy in many places.
The formation of desert lakes in northern Peru form a temporary habitat for
vegetation, freshwater fish and farmers.

• Is there an El Niño in the Atlantic Ocean?

There
are two kinds of El Niño phenomena in the Atlantic. One is a spin-off from the
Pacific El Niño, due to transmission through atmospheric fluctuations. This
atmospheric signal from the Pacific El Niño is the same phenomenon that causes
the Atlantic to experience fewer hurricanes during El Niño years. The result of
that "teleconnection" is that the tropical Atlantic usually
experiences a smaller but anomalous warming several seasons after the maximum
warming in the Pacific (which usually occurs in December).

The other
"Atlantic El Niño" effect is a non-synchronous and aperiodic warming
that occurs along the equator, entirely due to internal Atlantic dynamics. It
is only "El Niño-like" in the sense that those dynamics are similar
to the Pacific case, but it has no correlation with Pacific events. Moreover,
the magnitude of the warmings is much smaller, as is the typical period between
events. Hence, it is not of much consequence and should probably not be called
"El Niño", as this would create unnecessary confusion.

• Is it true that El Niño years are good wine years in
Europe?

Not
convincingly so. There is some statistical evidence that European winters may
be slightly more severe during El Niño. However, the consistency of this is
fairly low and there are numerous historical exceptions to the pattern. The
effects during the summer growing season are even harder to document.

Two
kinds of prediction are being made: (1) prediction of "equatorial warm
events" in the Pacific and (2) predictions of the impacts of those events.
The term "warm event" refers to a large scale warming in the tropical
Pacific and is frequently a euphemistic way of saying El Niño event or "El
Niño-like" without actually saying "El Niño". Sometimes these
events are too weak to be called "El Niño" in any universally agreed-
upon way, and to call it such in advance (given our poor ability to predict its
magnitude) would be to irresponsibly encourage unwarranted speculation about
its impacts, which will be minimal for the marginal cases. These predictions
are done routinely once or several times a year by a number of scientific
groups around the world, utilizing various kinds of numerical and/or
statistical models of how the oceans and atmosphere interact with each other.
Some of the predictions may be found through internet links to the World Wide
Web (WWW). NOAA/CPC provides the
U.S. "official" predictions. The International Research Institute
for Climate Prediction (IRI) disseminates predictions with a global
scope.

The
predictions of impacts are done more empirically and more informally, and
usually only in connection with a warm event that is already known to exist and
is strong enough to be called El Niño without controversy. A prediction of
impacts is typically based on what we know historically about climate responses
to (or correlations with) El Niño occurrences. It is a probabilistic statement
that certain climatic conditions can be expected more or less frequently than
normal due to the existence of El Niño conditions. Sometimes these predictions
are nothing more than the response of a scientist to a reporter or an
interviewer. The U. S. National Oceanic and Atmospheric Administration (NOAA)
routinely publishes formal
climate outlooks and advisories for the coming season (available
via the WWW and elsewhere). These are based on applied research by NOAA and
others. When a strong El Niño is in progress, these outlooks typically warn
about unusual conditions expected from El Niño.

•How far in advance can we predict the existence and the
intensity of an El Niño event?

Accurately?
In terms of timing and magnitude, hardly at all. In terms of saying that an
"equatorial warm event" (only vaguely defined in magnitude) will
start within a particular six-month time frame, we can typically foresee that
up to a year in advance. Some prediction models succeed in doing this on one
event and then go bust on the next. There's a good example of that in 1997.
Accordingly, we have come to rely more on a "consensus forecast" of
many models rather than on any one model. But the long-lead accuracy still
leaves a lot to be desired. Most models correctly anticipated a "warm
event" for the 1997/98 winter as early as one year earlier. However, none
of the predictions anticipated that strong anomalies would already be in place
by June, 1997, while their predicted magnitudes were small compared with what
actually occurred.

• Some have suggested that the dire predictions we heard
during the 1997 summer (once the El Niño was underway) were premature. Is that
true?

There
was a lot of misinformation and unfounded speculation, especially early in the
event when we had no accurate way of knowing how strong it would be. Although
the event indeed proved to be very strong, there was little basis at the time
for making those dire projections. But even within the parameters of those
projections the interpretations in the media were inaccurate, misleading and
unnecessarily alarming. The misinformation included things like the NY Times
saying (in July) that sea temperatures (not their anomalies) were as large or
larger than ever recorded. In fact, the measured temperatures on the equator
were well below previous El Niño highs; it was the difference between the
measured temperatures and what is normal for that time of year that was
extraordinary. The distinction, in terms of climate impacts, is crucial. The
prediction (in June 1997) that Californians would see a winter with several
times their normal rainfall did prove accurate, but no attempt was made to
prevent that projection from unnecessarily alarming people in places where no danger
was likely. When Reuters carried these news releases to places like Ecuador and
Panama (where strong El Niños really ARE disastrous), it caused a general
panic. Banks closed credits for crop planting and insurance companies refused
to offer crop insurance, even though under the direst scenario the severe
agricultural impacts could not occur until much later. Clearly, we have a long
way to go with how we handle ENSO-related information in the public eye.

• If the predictions of a watery grave are not reliable, can
we make any predictions at all?

Certainly.
We can say that "a warm event will occur" with enough advance notice
to be useful and that will probably prove to be correct in most instances. The
projections of climate impacts, given that an event will probably occur, are
more tenuous. Such projections are probabilistic, aimed at the center of our
expectations, not at the fringes of what is remotely possible, and they are
still not terribly accurate, though much better than anything we could do 20
years ago. If regional ENSO climate outlooks are correct 3-4 times out of every
five, then people who use them are ahead of the game over the long term.

•So, does our knowledge that an ENSO is occurring allow us
to predict when and where things like winter mudslides will occur?

No,
no more than we can with earthquakes. We can say their likelihood is greater,
over a season, and if summer brush fires have occurred in a particular hilly
location (making it more susceptible to mudslides), we can put out a danger
alert and perhaps take measures to offset that probability of occurrence.
Climate prediction is a game of likelihood, a way of rigging the coin toss so
that our call comes up better than 50:50. But there are no guarantees.

Nevertheless,
once the event is underway and we have the observations to tell us how strong
it is, we can then make some intelligent guesses about how it will develop, and
based on past experience we can make some fairly good projections about impacts
for the coming season. To whom those projections will prove useful will depend
on where they live and what time of year it is. By the time the next big event
comes around (10-15 years from now?) our advisories should be much better.

•What are the principal
changes that take place in the equatorial Pacific Ocean when El Niño occurs?

Before
El Niño occurs the Pacific trade winds blow steadily from east to west across
the Pacific. To maintain its balance the ocean must “lean into” the wind by
being higher in the west than in the east (like a person leaning windward in a
gale), thus creating a “pressure gradient force” toward the east in opposition
to the westward wind stress. Under these normal conditions the height of the
ocean surface in the western equatorial Pacific is greater than in the eastern
Pacific by a few tens of centimeters. The near surface temperature in the west
is about 10°C (20°F) greater than in the east and the thickness of the upper
warm layer of the ocean is about 120 meters in the west as opposed to only
30-40 meters in the east.

At the onset of El Niño conditions the trade
winds slacken across much of the basin. The eastward tilt of the sea surface is
then unbalanced and a series of ocean responses occur over a half-year period
that lead to a more nearly flattened sea surface with less east-west contrast
in temperatures and upper layer depths. The most noticeable change to an
observer is the remarkable increase in the equatorial sea surface temperatures
over the eastern half of the basin, which is normally much cooler.

•Why do the east-west
differences in sea level height initially exist and what happens to reduce
them?

Because
the upper layer of warm water is so much warmer and thicker in the west, the
density of a column of water there is less than in the east, where the water is
colder and denser. The volume in the west must therefore be greater and the
column height is higher. Under strong trade winds, characteristic ocean
circulations exist that maintain these density and volume differences. As the
winds fail the ocean circulations change so as to distribute the upper layer
water more evenly across the Pacific. The upper layer in the west becomes
thinner and in the east it becomes warmer and thicker. With the volumes and
densities being more equal the sea level difference is lessened or eliminated.

•Does this mean that the
warm water initially found off New Guinea in the western Pacific winds up off
the coasts of Peru and Ecuador?

No.
The process of adjustment is wavelike — more like a slosh in a bathtub or a
lava lamp. Parcels of water do not have to travel great distances in order to
reduce the upper layer thickness in the western Pacific or to expand the layer
in the east. Drifting buoys released by research vessels show that eastward
setting currents on the equator do move parcels toward South America but only
over a fraction of the basin width. If this seems strange, consider the
familiar experience of a person swimming just beyond the surf at a beach. As a
swell overtakes the swimmer, a surge of water carries the person only a short
distance toward the beach, yet the deformation of the water column and sea surface
(the wave crest) travels a comparatively immense distance.

•Then how does the water
off South America become so warm if not by importing warm water all the way
from the western Pacific warm pool?

The
eastern Pacific warms because El Niño suspends the local process normally
responsible for its extraordinary coolness. Because the upper layer in the
eastern Pacific is normally so thin, the colder water of the deep ocean is much
closer to the surface. The processes of mixing and upwelling due to wind action
then bring this available colder water to the surface, producing an impressive
10°C (20°F) cooling compared to elsewhere in the tropics.When El Niño causes the upper layer (in
the east) to thicken considerably, the underlying cool water is depressed to
greater depths and becomes unavailable to the upwelling and mixing, which are comparatively
shallow processes. The normal cooling fails to occur and we are left with a
warm anomaly.